It is well known in the art to add an inorganic filler to a cover stock as disclosed in JP-B 5-73427, JP-A 6-277312, 57-25867, 60-210272, and 61-290969.
In JP-A-6-277312, for example, an inorganic filler 15 such as titanium oxide or barium sulfate is added to a cover stock to shift the weight distribution within the ball from the core center toward the cover and to increase the inertia moment of the ball itself for preventing the spin of the ball in flight from attenuating. As a consequence, the ball becomes unsusceptible to an initial spin upon impact and travels a longer distance.
All these proposals intend to increase the specific gravity of the cover to increase the inertia moment of the ball for improving flight performance. However, excessive loading of the inorganic filler rather detracts from the resilience or rebound of the ball.
All these proposals intend to increase the specific gravity of the cover to increase the inertia moment of the ball for improving flight performance. However, excessive loading of the inorganic filler detracts from the resilience or rebound of the ball.
A variety of proposals have been made in order to improve the hitting feel of golf balls. Within the range of the prior art, a soft feel is obtained at the sacrifice of durability against consecutive strikes. It is very difficult to find a good compromise between hitting feel and durability against consecutive strikes.
An object of the invention is to provide a multi-piece solid golf ball having a good feel and improved durability against consecutive strikes.
In a first aspect, the invention provides a golf ball comprising a core, an intermediate layer consisting of one or more layers around the core, and a cover formed mainly of a thermoplastic resin around the intermediate layer. The core is formed mainly of a rubber base and has a deflection of at least 3.0 mm under an applied load of 100 kg. At least one layer of the intermediate layer contains 100 parts by weight of a resin component and 5 to 40 parts by weight of an inorganic filler.
Quite unexpectedly, the core made soft and the intermediate layer having an optimum amount of inorganic filler added cooperate in a synergistic manner so that the hitting feel of the ball becomes soft and pleasant and the durability against consecutive strikes and outer appearance of the ball are improved.
In a second aspect, the invention provides a golf ball comprising a core, an intermediate layer consisting of one or more layers around the core, and a cover formed mainly of a thermoplastic resin around the intermediate layer. The core is formed mainly of a rubber base and has a deflection of at least 3.0 mm under an applied load of 100 kg. At least one layer of the intermediate layer and the cover has added thereto an inorganic filler. The cover has a specific gravity of 1.0 to 1.3.
Also quite unexpectedly, the core made soft and the intermediate layer or cover having an optimum amount of inorganic filler added cooperate in a synergistic manner so that the hitting feel of the ball becomes soft and pleasant and the durability against consecutive strikes of the ball are improved without detracting from the outer appearance of the ball.
In a third aspect, the invention provides a golf ball comprising a core and a cover consisting of at least two layers around the core. The outermost layer of said cover contains 100 parts by weight of a cover stock composed mainly of a thermoplastic resin and 10 to 40 parts by weight of an inorganic filler.
In a fourth aspect, the invention provides a golf ball comprising a core, an intermediate layer around the core, and a cover consisting of at least two layers around the intermediate layer. The outermost layer of the cover contains 100 parts by weight of a cover stock composed mainly of a thermoplastic resin and 10 to 40 parts by weight of an inorganic filler.
The considerations the inventor made on golf balls are described here. (1) A golf ball undergoes a large deformation upon impact. As deformation and restitution are repeated, the ball gradually deforms and eventually fails. During the process, the durability against consecutive strikes of the cover plays a major role. There is a tendency that the harder the cover stock, the faster it fails. This phenomenon is similar to metal fatigue failure. Initially small cracks induced within the cover during molding are deformed and increased in size by consecutive strikes, eventually giving rise to a fatigue failure. (2) Another demand on the golf ball is long distance flight. A harder cover is desirable to this end. The cover contributes to the spin rate upon impact. A harder cover leads to a lower spin rate so that the ball will not sky high beyond the necessity, which is effective for increasing the flight distance. (3) When an inorganic filler is added for the purpose of increasing the flight distance, loading the cover with more inorganic filler is effective. However, a cover stock having a too large content of inorganic filler is likely to induce weld-marks and other defects in outer appearance when injection molded. It is thus desirable to reduce the filler loading of the cover.
Based on the above findings (1) to (3), the inventor made further investigations on a golf ball comprising a core and a cover consisting of at least two layers around the core or a golf ball comprising a core, an intermediate layer around the core and a cover consisting of at least two layers around the intermediate layer. The inventor has found that when the outermost layer of the cover is made of a cover stock based on a thermoplastic resin, 10 to 40 parts by weight of an inorganic filler is added to 100 parts by weight of the outermost layer cover stock, and preferably a spherical body excluding the cover outermost layer has a deflection of at least 3 mm under an applied load of 100 kg, quite unexpectedly, the cover is prevented from fatigue failure without detrimental effect on its outer appearance. There is obtained a multi-piece golf ball exhibiting improved durability against consecutive strikes and a soft pleasant feel when hit.
Unlike the prior art approach of adding an inorganic filler to the cover stock to increase its specific gravity for thereby increasing the inertia moment of the ball and improving the flight performance thereof, the present invention intends to provide a golf ball having a good feel and improved durability against consecutive strikes, which have never been achieved in the prior art approach, by combining a soft core with an intermediate layer and/or a cover each consisting of at least one layer and containing an appropriate amount of uniformly dispersed inorganic filler so that the core and the intermediate layer and/or over cooperate in a synergistic manner.